Non-aqueous electrolyte secondary battery and battery module

ABSTRACT

A non-aqueous electrolyte secondary battery including an electrode assembly, a non-aqueous electrolyte, and a substantially rectangular battery case for housing the electrode assembly and the non-aqueous electrolyte. The thickness α, the width β, and the height γ of the battery case satisfy the relation α&lt;β≦γc. The electrode assembly includes a positive electrode, a negative electrode, and a porous heat-resistant layer disposed between these electrodes. The positive electrode includes a positive electrode active material layer, and the negative electrode includes a negative electrode active material layer. The ratio of the pore volume included in a predetermined area of the porous heat-resistant layer to the battery theoretical capacity is 0.18 to 1.117 ml/Ah. The predetermined area has the same area as the positive electrode active material layer. The porosity of the porous heat-resistant layer is 35 to 85%.

RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.11/442,983, filed on May 31, 2006, now U.S. Pat. No. 7,951,482, claimingpriority of Japanese Application No. 2005-158709, filed on May 31, 2005,the disclosures of which Applications are incorporated by referenceherein.

FIELD OF THE INVENTION

The present invention relates to non-aqueous electrolyte secondarybatteries and battery modules using such batteries. More particularly,the present invention relates to a non-aqueous electrolyte secondarybattery in which an electrode assembly can keep a sufficient amount of anon-aqueous electrolyte even when the change in the electrode assemblysize is restrained, and to a battery module using such batteries.

BACKGROUND OF THE INVENTION

Non-aqueous electrolyte secondary batteries, particularly, lithium ionsecondary batteries have a high operating voltage and a high energydensity. Thus, development for lithium ion secondary battery has beenspurred recently, as a power source for electrically-powered tools orvehicles which necessitate high output, in addition to a power sourcefor driving portable electronic devices such as mobile phones, laptopcomputers, and camcorders. Particularly, high capacity lithium ionsecondary batteries have been developed actively as a power source forreplacing available nickel-metal hydride storage batteries to be usedfor hybrid electric vehicles (HEV).

The power source for HEVs needs to have a higher capacity, compared withthe power source for small household devices. Battery modules includinga plurality of substantially rectangular batteries stacked arepreferably used for a power source for HEVs, since a high capacity canbe obtained even with a small size module.

In such battery modules, batteries tend to be affected by a bindingforce for retaining the module dimension, especially the batteriespositioned in the center area. Under such effects from the bindingforce, in non-aqueous electrolyte secondary batteries using micro-porousfilm comprising resin such as polyolefin as the separator, for example,when a pressure is applied to the battery, the non-aqueous electrolyteis easily forced out from the separator. In fact, when a substantiallyrectangular lithium ion secondary battery using a separator made ofresin is molded with resin, the binding force by the molding resinforces the non-aqueous electrolyte out from the separator. As a result,ion conductivity is lost in the separator to decline the batteryperformance.

There has been proposed to use a highly stiff porous heat-resistantlayer comprising a nonconductive filler such as silica and a binder suchas polyvinylidene fluoride, instead of a conventional resin-madeseparator (Japanese Laid-Open Patent Publication No. Hei 10-106530).

In battery modules, several tens of substantially rectangularnon-aqueous electrolyte secondary batteries are usually bound. Theelectrodes expand when the batteries are charged, the battery case triesto expand. However, in battery modules, each battery cannot freelychange its shape since all the batteries are bound, and the force causedby the battery case deformation is likely to concentrate on the centerof the module. Thus, in the battery positioned at around the center ofthe module, a load of 100 kgf/cm² (average 20 kgf/cm²) at the maximum isapplied at the ends of the battery in the thickness direction thereofwhile charging.

The inventors of the present invention found in their examination thatunder such harsh environment, the problem of the shortage of thenon-aqueous electrolyte in the porous heat-resistant layer cannot besolved just by using the technique disclosed in Japanese Laid-OpenPatent Publication No. Hei 10-106530.

The present invention was made in view of the above problems, and aimsto provide a reliable non-aqueous electrolyte secondary battery whichcan avoid the forcing out of the non-aqueous electrolyte from theelectrode assembly, even under the environment where the size change inthe battery case is unacceptable.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a non-aqueous electrolyte secondarybattery comprising an electrode assembly, a non-aqueous electrolyte, anda substantially rectangular battery case for housing the electrodeassembly and the non-aqueous electrolyte. A thickness α, a width β, anda height γ of the battery case satisfy the relation α<β≦γ. The electrodeassembly includes a positive electrode, a negative electrode, and aporous heat-resistant layer disposed between the positive electrode andthe negative electrode. The positive electrode includes a positiveelectrode active material layer and the negative electrode includes anegative electrode active material layer. The ratio of a pore volumeincluded in the predetermined area of the porous heat-resistant layer tothe battery theoretical capacity is 0.18 to 1.117 ml/Ah. The porosity ofthe porous heat-resistant layer is 35 to 85%. The predetermined area hasthe same area as the positive electrode active material layer.

The non-aqueous electrolyte secondary battery preferably furtherincludes a separator comprising resin disposed between the positiveelectrode and the negative electrode. The ratio of a thickness B of theporous heat-resistant layer to a thickness A of the separator, B/A, is0.35 to 2.

The porous heat-resistant layer is preferably attached to at least oneof the positive electrode active material layer and the negativeelectrode active material layer.

The porous heat-resistant layer preferably includes a nonconductivefiller and a binder. The nonconductive filler preferably includes atleast one selected from the group consisting of alumina, silica,magnesia, titania, and zirconia. The median size of the nonconductivefiller is preferably 0.3 to 4 μm.

The binder preferably includes at least one of polyvinylidene fluorideand modified acrylic rubber. The amount of the binder is preferably 0.3to 8.5 parts by weight per 100 parts by weight of the nonconductivefiller.

When the porous heat-resistant layer is attached to one of the positiveelectrode active material layer and the negative electrode activematerial layer, the surface roughness of the active material layer ofthe electrode to which porous heat-resistant layer is not attached ispreferably larger than the surface roughness of the porousheat-resistant layer.

The present invention also relates to a battery module comprising:

(a) a stack in which at least two of the above non-aqueous electrolytesecondary batteries are stacked in at least a thickness directionthereof;

(b) end plates placed at both ends of the stack, the both ends being theends in the thickness direction of the battery; and

(c) at least two bridges for binding the at least two non-aqueouselectrolyte secondary batteries by connecting the two end plates.

The ratio of a thickness Y of the end plate to a thickness α of the caseof the battery, Y/α, is preferably 0.4 to 2.

While the novel features of the invention are set forth particularly inthe appended claims, the invention, both as to organization and content,will be better understood and appreciated, along with other objects andfeatures thereof, from the following detailed description taken inconjunction with the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a schematic vertical cross sectional view of a portion of anon-aqueous electrolyte secondary battery according to an embodiment ofthe present invention.

FIG. 2 is a perspective view of an example of a substantiallyrectangular battery case.

FIG. 3 is a schematic vertical cross sectional view showing a portion ofa non-aqueous electrolyte secondary battery according to anotherembodiment of the present invention.

FIG. 4 is a perspective view of a battery module of an embodiment of thepresent invention.

FIG. 5 is a perspective view of a battery module of another embodimentof the present invention.

FIG. 6 is a perspective view of a battery module of still anotherembodiment of the present invention.

FIG. 7 is a perspective view of a battery module of still anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Embodiment 1

FIG. 1 schematically shows a non-aqueous electrolyte secondary batteryof an embodiment of the present invention.

The battery of FIG. 1 includes an electrode assembly including apositive electrode 1 and a negative electrode 2, a non-aqueouselectrolyte (not shown), and a battery case 4 for housing the assemblyand the electrolyte. A porous heat-resistant layer 3 disposed betweenthe positive electrode 1 and the negative electrode 2. As shown in FIG.2, the battery case 4 has a substantially rectangular shape. A thicknessα, a width β, and a height γ of the battery case 4 (or the non-aqueouselectrolyte secondary battery) satisfy a relation α<β≦γ. FIG. 2 onlyshows the shape of the battery case, and a positive electrode terminal,a negative electrode terminal, and the like are not shown.

The positive electrode 1 has a positive electrode active material layerincluding an active material, a binder, and a conductive agent. For thepositive electrode active material, for example, LiMO₂ (M is at leastone selected from the group consisting of Co, Ni, Mn, Al, and Mg) orLiMn₂O₄ may be used.

Although the positive electrode binder is not particularly limited,examples include polyvinylidene fluoride, polytetrafluoroethylene, andparticulate modified acrylic rubber (polyacrylic acids) (BM-500 (productname) manufactured by Zeon Corporation). Polytetrafluoroethylene andparticulate modified acrylic rubber are preferably used in combinationwith a thickener such as for example, carboxymethyl cellulose,polyethylene oxide, and modified acrylic rubber (polyacrylonitriles)(BM-720H (product name) manufactured by Zeon Corporation) soluble in thesolvent used when the positive electrode active material layer is made.

The amount of the positive electrode binder included in the positiveelectrode active material layer is preferably 1 to 8 parts by weight per100 parts by weight of the positive electrode active material. When thethickener is to be included, the amount of the thickener is preferably 1to 4 parts by weight per 100 parts by weight of the positive electrodeactive material.

For the positive electrode conductive agent, for example, acetyleneblack, Ketjen Black, and various graphites may be used. These can beused singly or in combination.

The amount of the positive electrode conductive agent included in thepositive electrode active material layer is preferably 1.5 to 8 parts byweight per 100 parts by weight of the positive electrode activematerial.

The positive electrode 1 may be formed of a positive electrode currentcollector and the positive electrode active material layer carriedthereon. The positive electrode current collector is preferably a metalfoil such as aluminum.

The negative electrode 2 comprises a negative electrode active materiallayer including an active material and a binder. For the negativeelectrode active material, various natural graphites, various artificialgraphites, silicon-containing composite, various alloy materials may beused.

For the negative electrode binder, for example, rubber polymer includinga styrene unit and a butadiene unit may be used. For example,styrene-butadiene copolymer, acrylic acid-modified styrene-butadienecopolymer or the like may be used, although not limited thereto.

The negative electrode binder is preferably used in combination with athickener comprising water-soluble polymer. For the water-solublepolymer, a cellulose resin is preferable, especially carboxymethylcellulose.

The amount of the negative electrode binder included in the negativeelectrode active material layer is preferably 0.1 to 5 parts by weightper 100 parts by weight of the negative electrode active material. Whenthe thickener is to be included, the amount of the thickener ispreferably 0.1 to 5 parts by weight per 100 parts by weight of thenegative electrode active material.

The negative electrode 2 may also be formed of a negative electrodecurrent collector and the negative electrode active material layercarried thereon. The negative electrode current collector is preferablya metal foil such copper.

The electrode assembly may be a stack-type or a wound-type. Thestack-type electrode assembly may be formed for example by stacking arectangular positive electrode 1 and a rectangular negative electrode 2with a porous heat-resistant layer interposed therebetween. Thewound-type electrode assembly may be formed by winding a sheet positiveelectrode 1 and a sheet negative electrode 2 with a porousheat-resistant layer disposed therebetween, so that the cross sectionforms a substantially rectangular shape.

The non-aqueous electrolyte includes a non-aqueous solvent, and a solutedissolved therein. For the non-aqueous solvent, without particularlimitation, ethylene carbonate, ethyl methyl carbonate, propylenecarbonate, dimethyl carbonate, diethyl carbonate, or the like may beused. These may be used singly or may be used in combination.

For the solute, lithium salt such as LiPF₆, LiBF₄ or the like ispreferably used.

The non-aqueous electrolyte may include vinylene carbonate,cyclohexylbenzene, or derivatives thereof. By including such solvents inthe non-aqueous electrolyte, a coating derived from the solvent isformed on the active material surface of the positive electrode and/orthe negative electrode. Such coating enables for example assurance ofbattery stability under over-charged state.

In the present invention, the porous heat-resistant layer 3 has aporosity of 35 to 85%. To improve ion conductivity of the porousheat-resistant layer 3, the porosity has to be set higher. However, withan excessively higher porosity, the strength of the porousheat-resistant layer 3 decreases. By setting the porosity of the porousheat-resistant layer 3 to the above range, ion conductivity of theporous heat-resistant layer 3 is improved, while maintaining a higherstrength level of the porous heat-resistant layer. The porosity of theporous heat-resistant layer 3 can be adjusted for example by changingthe median size of the nonconductive filler and drying conditions. Forexample, the drying temperature may be set high, or the quantity of warmair supply may be increased to increase the porosity of the porousheat-resistant layer.

The ratio of a pore volume V included in a predetermined area of theporous heat-resistant layer to theoretical capacity C of the battery,V/C, is 0.18 to 1.117 ml/Ah. The predetermined area of the porousheat-resistant layer has the same area as the area of the positiveelectrode active material layer. For example, the pore volume V is thevolume of the pores included in a portion of the porous heat-resistantlayer where the face thereof contacting the positive electrode activematerial layer has the same area as the positive electrode activematerial layer. The area of the positive electrode active material layerrefers to the area of a side of the positive electrode active materiallayer opposite to the side contacting the positive electrode currentcollector.

When a plurality of porous heat-resistant layers are included in thebattery, the total of the pore volume included in the predetermined areaof each porous heat-resistant layer is the pore volume V.

For example, when the porous heat-resistant layer is provided on bothsides of one electrode, the total of the pore volume included in thepredetermined area of the porous heat-resistant layers provided on bothsides of the electrode is the pore volume V. The pore volume of eachporous heat-resistant layer is V/2. The ratio of the V/2 to C, (V/2)/C,is preferably 0.0889 to 0.57 ml/Ah.

By setting the ratio of the pore volume V to the theoretical capacity Cof the battery to 0.18 to 1.117 ml/Ah, the distribution of thenon-aqueous electrolyte in the porous heat-resistant layer is optimized,and battery performance can be kept to a higher level. On the otherhand, when the pore volume of the porous heat-resistant layer 3 is morethan 1.117 ml/Ah, pores without the non-aqueous electrolyte chargedoccupy more space in the porous heat-resistant layer, to decline batteryperformance. When the pore volume of the porous heat-resistant layer 3is less than 0.18 ml/Ah, the amount of the non-aqueous electrolytesufficient for the battery to function cannot be maintained when a loadis applied to the ends of the battery in the thickness directionthereof, while the battery is in charged state.

The thickness of the porous heat-resistant layer is preferably 2 to 20μm. When the thickness of the porous heat-resistant layer is below 2 μm,the ratio V/C does not fall within the above preferable range, as longas the positive electrode is made thin. Thus, the theoretical capacityper unit volume of the battery becomes excessively small. When thethickness of the porous heat-resistant layer is over 20 μm, the ratioV/C does not fall within the preferable range, as long as the positiveelectrode is made thicker. Thus, a high output cannot be obtained.

As described above, by adjusting the ratio V/C to fall within the aboverange, the porous heat-resistant layer can keep its shape during thebattery usage, even though a load of 17 to 100 kgf/cm² is applied to theends of the battery in the thickness direction thereof (area β×γ).Further, the porous heat-resistant layer can keep the amount of thenon-aqueous electrolyte sufficient for the battery to function, even aforce generated by restraining the electrode assembly deformation (forexample, a size restraints at the time of making a battery into abattery module) is applied on the battery.

Thus, the porous heat-resistant layer used in the present invention hasmany pores capable of absorbing the non-aqueous electrolyte, and has ahigher rigidity. Thus, the porous heat-resistant layer is capable ofkeeping the sufficient amount of the non-aqueous electrolyte, inaddition to having durability against the force for restraining theelectrode assembly deformation while charging. Thus, even when a load isapplied to the battery including the above porous heat-resistant layer,the non-aqueous electrolyte can be prevented from being squeezed outfrom the porous heat-resistant layer. Therefore, even with therestrictions on changing the size of the non-aqueous electrolytesecondary battery, cycle life characteristic can be made excellent. Sucheffect becomes notable especially in the case of substantiallyrectangular non-aqueous electrolyte secondary batteries that arearranged in large number in a small space.

Therefore, the non-aqueous electrolyte secondary battery of the presentinvention can maintain excellent cycle life characteristic and highreliability even when used as a power source for HEVs which require ahigh dimensional precision.

The porosity of the porous heat-resistant layer can be obtained, forexample, from the thickness of the porous heat-resistant layer, the truespecific gravity of the nonconductive filler and the binder, and theweight ratio between the nonconductive filler and binder. The thicknessof the porous heat-resistant layer can be obtained for example bycutting the porous heat-resistant layer and determining the thickness at10 points at the cut surface with an electron microscope. The average ofthe determined values is regarded as the thickness of the porousheat-resistant layer.

The pore volume V of the porous heat-resistant layer can be obtained bymultiplying the volume of the porous heat-resistant layer having thesame area as the positive electrode active material layer by theporosity.

The porous heat-resistant layer is preferably attached on at least oneof the positive electrode and the negative electrode, for example, onthe active material layer of the electrode. By attaching the porousheat-resistant layer on the electrode, the structural strength of theporous heat-resistant layer can be kept to a higher level. The batterycapacity of usual secondary batteries are regulated by a positiveelectrode. That is, the size of the negative electrode active materiallayer is made larger than that of the positive electrode active materiallayer. In such a case, the porous heat-resistant layer is preferablyattached at least on the negative electrode active material layer, inview of preventing a short circuit between the positive electrode andthe negative electrode.

The porous heat-resistant layer may be formed of a nonconductive filleras a main material and a binder for binding the nonconductive fillerparticles. The porous heat-resistant layer may be a porous sheetcomprising a highly heat-resistant resin. For the highly heat-resistantresin, for example, aramid and polyamideimide with a melting temperatureof 250° C. or more may be mentioned.

For the material forming the nonconductive filler, for example,heat-resistant resin and organic oxide may be mentioned. When thenonconductive filler comprises organic oxide, the nonconductive fillerpreferably includes at least one selected from the group consisting ofalumina, silica, magnesia, titania, and zirconia. This is because theseorganic oxides are high in heat-conductivity and electrochemicalstability. When the nonconductive filler comprises the heat-resistantresin, for the nonconductive filler, for example, beads comprising theheat-resistant resin may be used.

For the nonconductive filler, the nonconductive filler of various formsmay be used. However, the median size of the nonconductive filler ispreferably 0.3 to 4 μm. When the porous heat-resistant layer is formedfrom a nonconductive filler and a binder, the gaps formed betweennonconductive filler particles function as pores that become movingpaths for ions. When the nonconductive filler has an excessively smallmedian size, the nonconductive filler is so densely charged that thepore volume in the porous heat-resistant layer becomes small. On theother hand, when the median size of the nonconductive filler isexcessively large, the nonconductive filler is so roughly charged thatthe strength of the porous heat-resistant layer cannot be made higher.Thus, by setting the median size of the nonconductive filler to 0.3 to 4μm, a porous heat-resistant layer with an appropriate pore volume and ahigher strength can be formed.

The binder included in the porous heat-resistant layer preferablyincludes at least one of polyvinylidene fluoride and acrylic rubber.Binders generally absorb non-aqueous electrolytes and swell afterbatteries are formed. Thus, the amount of the binder to be added ispreferably small. Since the polyvinylidene fluoride and the acrylicrubber mentioned above exhibit binding effects even with a small amount,the amount to be added can be made small.

For the acrylic rubber, for example, particulate modified acrylic rubber(BM-500B (product name) manufactured by Zeon Corporation) and modifiedacrylic rubber (BM-720H (product name) manufactured by Zeon Corporation)that is soluble in the solvent used for the paste for manufacturing theporous heat-resistant layer may be mentioned.

When polyvinylidene fluoride is used for the binder, an appropriateviscosity can be given to the paste for making the porous heat-resistantlayer. Thus, a homogenous porous heat-resistant layer can be formed. Theabove-mentioned particulate modified acrylic rubber is preferably usedin combination with a thickening binder, particularly, polyvinylidenefluoride, carboxymethyl cellulose, polyethylene oxide, and a modifiedacrylic rubber soluble in the above-mentioned solvent.

The amount of the binder included in the porous heat-resistant layer ispreferably 0.3 to 8.5 parts by weight per 100 parts by weight of thenonconductive filler. As mentioned in the above, binders swell byabsorbing non-aqueous electrolytes after batteries are formed, and as aresult, pore diameters in the porous heat-resistant layer become small.Thus, ion conductivity of the porous heat-resistant layer may decrease.Thus, although the smaller the amount of the binder to be added, thebetter, when the amount of the binder to be added is excessively small,the strength of the porous heat-resistant layer is reduced. By settingthe amount of the binder within the above range, the porousheat-resistant layer with appropriate ion conductivity and high strengthcan be obtained.

A manufacturing method of a porous heat-resistant layer including anonconductive filler and a binder is explained next.

A nonconductive filler and a binder are mixed with a predetermineddispersion medium or solvent. The obtained mixture is stirred by forexample a double-armed kneader to obtain a paste. This paste is appliedon an electrode or on a metal plate from which a formed porousheat-resistant layer easily separates, by a doctor blade or a diecoater, and the applied paste is dried with a far-infrared radiation ora hot blast. The porous heat-resistant layer is thus formed.

When the porous heat-resistant layer is attached on the active materiallayer of one of the positive electrode and the negative electrode, thesurface roughness of the active material layer of the other electrode onwhich the porous heat-resistant layer is not attached is preferablylarger than the surface roughness of the porous heat-resistant layer.The ratio of a surface roughness Ra1 of the porous heat-resistant layerand a surface roughness Ra2 of active material layer of the otherelectrode on which the porous heat-resistant layer is not attached ispreferably 1:2 to 1:8. The porous heat-resistant layer is highly stiffbut fragile. Therefore, when the battery is dropped, the porousheat-resistant layer may be damaged or the relative position of thepositive electrode and the negative electrode may be misaligned. On theother hand, by making the surface roughness of the active material layerlarge, the anchor effect causes the porous heat-resistant layer to becaught into the active material layer to which the porous heat-resistantlayer is not attached. Thus, battery durability can be improved againstproblems such as dropping.

As shown in FIG. 3, a separator 5 comprising resin can be disposedbetween the positive electrode 1 and the negative electrode 2, otherthan the porous heat-resistant layer 3. In FIG. 3, the porousheat-resistant layer 3 is formed on the negative electrode 2. In FIG. 3,the same reference numerals are given to the same components as in FIG.1.

By further disposing the separator comprising resin between the positiveelectrode and the negative electrode, the highly stiff but fragileporous heat-resistant layer can be protected by the separator. Thus,durability of the battery can be improved against problems such aspeeling off of the porous heat-resistant layer from the electrode.Further, even when the porous heat-resistant layer is peeled off, withthe separator comprising resin disposed between the positive electrodeand the negative electrode, a short circuit between the positiveelectrode and the negative electrode can be prevented.

The separator 5 is preferably a micro-porous film comprising a resinwith a melting point of 200° C. or less. Particularly, the materialforming the separator is further preferably polyethylene, polypropylene,a mixture of polyethylene and polypropylene, or a copolymer including anethylene unit and a propylene unit. The separator 5 may include a fillerand the like. The resin component is preferably 50 to 100 wt % of theseparator.

By forming the separator 5 from the above materials, in the case of thebattery external short circuit, the heat caused by the external shortcircuit melts the separator 5 to eliminate the pores exist in theseparator. This increases the battery resistance, and decreases theshort circuit current. Thus, even in the case of the battery externalshort circuit, the battery can be prevented from having a hightemperature from the heat caused.

When the battery further includes a separator, the ratio of a porousheat-resistant layer thickness B to a separator thickness A, B/A, ispreferably 0.35 to 2. The porous heat-resistant layer thickness B refersto the thickness of one porous heat-resistant layer.

When the dimensional change in the battery case is restricted, theseparator cannot exhibit its ability to keep the non-aqueous electrolytesufficiently. Thus, when the separator is excessively thick relative tothe porous heat-resistant layer, the separator becomes a mere resistantcomponent for battery reaction. On the other hand, when the ratio B/A isgreater than 2, the effects noted in the above cannot be exhibitedeasily. Thus, by setting the ratio B/A within the above range, peelingoff or the like of the porous heat-resistant layer can be prevented,while decreasing resistance in battery reaction.

The thickness of the separator 5 is preferably 10 to 40 μm in view ofsecuring ion conductivity while keeping the energy density.

The material forming the battery case may be a metal material or alaminate film. The metal material includes for example iron andaluminum. When the battery case is formed of a metal plate comprisingthe above metal material, the thickness of the metal plate is preferably100 to 500 μm. When the battery case is made of iron, the inner portionof the battery case is preferably nickel-plated. The laminate filmincludes a film comprising a three layers made up of for example apolyamide layer, an aluminum layer, and a polyethylene layer. Thethickness of the laminate film is preferably 50 to 200 μm.

Embodiment 2

With reference to FIGS. 4 to 7, a battery module formed from a pluralityof non-aqueous electrolyte secondary batteries of the present inventionis explained.

A battery module of the present invention comprises:

(a) a stack in which at least two of the above non-aqueous electrolytesecondary batteries are stacked in at least the thickness direction ofthe battery;

(b) end plates placed on both ends of the stack, the ends being the endsin the thickness direction of the battery; and

(c) at least two bridges for binding the at least two batteries byconnecting the two end plates.

As an example, FIG. 4 shows a battery module in which the abovenon-aqueous electrolyte secondary batteries are stacked in the thicknessdirection of the battery.

The battery module 40 in FIG. 4 comprises a stack 42 in which aplurality of the above non-aqueous electrolyte secondary batteries 41are stacked in the thickness direction of the battery.

On first and second ends of the stack 42, end plates 44 and 45 areplaced. The first and second ends refer to the ends of the stack in thebattery thickness direction. The end plates 44 and 45 are connected byfour bridges 46. In FIG. 4, two bridges are contacting third end of thestack. The third end here refers to the one end in the battery widthdirection. The other two bridges (not shown) are contacting fourth endopposite to the third end. One bridge may be provided at each of thethird and fourth ends. Alternatively, at least one bridge may be placedon each of the four planes of the stack where the end plate is notplaced to connect the two end plates.

Each battery 41 is connected in series by a connective terminal 43. InFIG. 4 as well, the positive electrode terminal and the negativeelectrode terminal provided at the battery 41 are not shown. When thebattery case is formed from an insulator such as a laminate sheet, theseterminals do not have to be insulated from the battery case.

When conventional non-aqueous electrolyte secondary batteries are usedto form the battery module as in the above, repeated charge anddischarge cause non-aqueous electrolyte to be squeezed out from theelectrode assembly of the battery placed in the center of the batterymodule. Thus, the battery placed in the center of the battery moduledeteriorates to a greater degree, and battery performance varies amongthe batteries in the battery module. The variation in batteryperformance of each battery causes cycle life characteristic of thebattery module to drastically drop. On the other hand, in thenon-aqueous electrolyte secondary battery of the present invention, evenunder pressure, the porous heat-resistant layer can sufficiently keepthe non-aqueous electrolyte. Thus, the battery module using thenon-aqueous electrolyte secondary battery of the present invention canavoid a sudden drop in cycle life characteristic, unlike battery modulesusing conventional non-aqueous electrolyte secondary batteries.

The ratio of a thickness Y of the end plate to a thickness α of thenon-aqueous electrolyte secondary battery (i.e., a thickness of thebattery case), Y/α, is preferably 0.4 to 2. The thickness of the endplate is in proportion to the force to restrain the electrode assemblydeformation. The thicker the end plate, the easier to keep the electrodeassembly to a predetermined size. However, when the end plate isexcessively thin, the electrode assembly cannot be kept to apredetermined size. When the end plate is excessively thick, thedistortion from the electrode assembly deformation cannot even be easedbetween the stacked non-aqueous electrolyte secondary batteries. Thus,by setting the ratio of the end plate thickness Y to the batterythickness α, Y/α, within the above range, distortion from the electrodeassembly deformation can be eased between the non-aqueous electrolytesecondary batteries, while keeping the battery module to a predeterminedsize.

For the material forming the end plates 44 and 45, and the bridge 46,metals such as aluminum, an aluminum alloy, steel, and stainless steelare preferably used in view of strength and durability. In the batterymodule, the material forming the end plate and the material forming thebridge may be the same or different.

The bridge may be connected to the end plate by using screws, or bywelding.

In the stack included in the battery module, as shown in FIG. 5, thenon-aqueous electrolyte secondary batteries may be stacked in the widthdirection thereof, in addition to the thickness direction thereof.

A battery module 50 in FIG. 5 comprises a stack 52, in which thenon-aqueous electrolyte secondary batteries 51 are stacked in the widthdirection and the thickness direction thereof. As in the above, eachbattery 51 is connected by a connective terminal 53 in series.

At both ends of the stack 52 (the ends being the ends in the batterythickness direction), end plates 54 and 55 are placed. Four bridges 56are also placed, as in the battery module in FIG. 4.

Further, in the stack 52, when the non-aqueous electrolyte batteries areplaced in the width direction thereof, in addition to the thicknessdirection thereof, as shown in FIGS. 6 and 7, a connector may beprovided for further connecting center parts of the two end plates. InFIGS. 6 and 7, the same reference numerals are used for the samecomponents in FIG. 5.

In a battery module 60 in FIG. 6, other than bridges 56, two connectors66 going through between a first row 62 and a second row 63 of thesecondary batteries 51 stacked in the thickness direction thereof forconnecting center parts of two end plates 64 and 65 are provided. Theseconnectors 66 are placed in line with the height direction of the endplate.

In the battery module in FIG. 6, the connector 66 comprises a threadedshaft 66 a and a nut 66 b, and the threaded shaft is fixed on the endplate by the nut.

Each battery 51 is connected by a connective terminal 53.

When the batteries are stacked in the battery width direction as well inthe stack, the width of the stack also increases. At this time, when theend plates placed in the ends of the stack in the battery thicknessdirection are connected by bridges placed in both ends of the stackwidth direction, the center parts of the end plates are bent towards theoutside from the stack, which may cause an insufficient binding of thestack at the center parts of the end plates. As shown in FIG. 6, byfurther providing connectors for connecting the center parts of two endplates, the bent in the center part of the end plate can be reduced.Thus, the plurality of batteries included in the stack can be bound bytwo end plates sufficiently with even force.

Further, since the center parts of the two end plates can be connectedfor example by using only threaded shafts and nuts, the connectionbetween the two end plates can be done easily for a low cost.

The connection between the center parts of the two end plates can alsobe done as shown in FIG. 7.

In a battery module 70 of FIG. 7, two end plates 54 and 55 are connectedby a connector 71 for connecting the top edges of the end plates, and aconnector 72 for connecting the lower edges of the end plates, otherthan the bridges 56. The top edges and the lower edge are parallel tothe width direction of the end plate.

With the structure shown in FIG. 7, the plurality of batteries includedin the stack can be sufficiently bound with even force by the two endplates, as in the battery module in FIG. 6. Further, since the space forthe connector to go through between the first row 73 and the second row74 does not have to be provided in the battery module in FIG. 7, spaceefficiency is high compared with the battery module in FIG. 6. Eachbattery 51 is connected by connective terminal 53, as in the above.However, the battery 51 b included in the first row 73 and the battery51 c included in the second row 74 are connected by a wire 75 goingthrough the top of the connector.

For the connectors 71 and 72 used in the battery module 70, the abovebridge may be used.

In such case as well, the ratio of the end plate thickness Y to thebattery thickness α, Y/α, is preferably 0.4 to 2.

In the above stack, the number of the non-aqueous electrolyte secondarybattery to be stacked in the thickness direction thereof is preferably 2to 30, and the number of the batteries to be stacked in the widthdirection thereof is 1 to 2.

In the following, the present invention is explained in detail based onExamples. Although a cylindrical battery comprising a wound-typeelectrode assembly was made in Examples, the present invention may beapplied to for example a rectangular battery comprising the wound-typeelectrode assembly or a stack-type electrode assembly.

EXAMPLE 1

Preparation of Positive Electrode

A positive electrode paste was prepared by mixing 30 kg of LiCoO₂, 10 kgof N-methyl-2-pyrrolidone (MNP) solution of polyvinylpyrrolidone (#1320manufactured by Kureha Corporation, solid content 12 wt %), 900 g ofacetylene black, and an appropriate amount of NMP with a double-armedkneader. The paste was applied on both sides of an aluminum foil with athickness of 15 μm, dried, and pressed so that a total thickness of thefoil with a formed active material layer was 120 μm, to obtain apositive electrode plate. The obtained positive electrode plate was cutso that the active material layer has a width of 54 mm and a length of338 mm, to obtain a positive electrode. In the obtained positiveelectrode, an area of the active material layer per side of the positiveelectrode was 183 cm².

Preparation of Negative Electrode

A negative electrode paste was prepared by mixing 20 kg of artificialgraphite, 750 g of acrylic acid modified styrene-butadiene copolymer(BM-400B manufactured by Zeon Corporation, solid content 40 wt %), 300 gof carboxymethyl cellulose, and an appropriate amount of water with adouble-armed kneader. This paste was applied on both sides of a copperfoil with a thickness of 10 μm, dried, and pressed so that a totalthickness of the foil with a formed active material layer was 132 μm, toobtain a negative electrode plate. The obtained negative electrode platewas cut so that the active material layer has a width of 58 mm and alength of 408 mm, to obtain a negative electrode.

Preparation of Porous Heat-Resistant Layer

An aramid sheet with a thickness of 14 μm was obtained by carrying out apaper-making process for fibrous aramid resin with a 1000-mesh sieve.The sheet was heated for an hour with a temperature of 270° C., for astructural reinforcement and an adjustment of its porosity to 60%.Afterwards, the sheet was cut to give the same size as the negativeelectrode active material layer, to obtain a porous heat-resistantlayer.

The porosity of the porous heat-resistant layer was obtained as noted inthe above. The pore volume was obtained by multiplying the volume of theporous heat-resistant layer having the same area as the positiveelectrode active material layer, by the porosity of the porousheat-resistant layer.

Battery Assembly

Thus obtained positive electrode, negative electrode, and porousheat-resistant layer disposed between the positive electrode and thenegative electrode were wound to give a substantially rectangular form,thereby obtaining an electrode assembly. At this time, an exposedportion of the aluminum foil where the positive electrode activematerial layer was not provided was arranged to be on top of theelectrode assembly. An exposed portion of the copper foil where thenegative electrode active material layer was not provided was arrangedto be on bottom of the electrode assembly.

To the exposed portion of the aluminum foil, a positive electrodecurrent collecting plate (thickness 0.3 mm) of aluminum was welded, andto the exposed portion of the copper foil, a negative electrode currentcollecting plate (thickness 0.3 mm) of copper was welded.

Then, the electrode assembly was housed in a substantially rectangularbattery case with a thickness of 5 mm, a width of 42 mm, and a height of71 mm. For the battery case material, a laminate film with a thicknessof 70 μm was used. The laminate film is formed of a polyethylene layer(20 μm in thickness), an aluminum layer (30 μm in thickness), and apolyamide layer (30 μm in thickness). In the laminate film, the layersare laminated from the inner side to the outer side of the battery case,in the order of the polyethylene layer, the aluminum layer, and thepolyamide layer.

Subsequently, into the battery case, 4 ml of a non-aqueous electrolyteincluding a solvent mixture of ethylene carbonate and ethyl methylcarbonate (volume ratio 1:3), and LiPF₆ dissolved in the solvent mixturewas injected. The concentration of LiPF₆ was set to 1.0 mol/L.

Then, by sealing an opening of the battery case, a lithium ion secondarybattery with a substantially rectangular shape was made. The obtainedbattery was regarded as the battery of Example 1. The theoreticalcapacity of the obtained battery was 860 mAh. The capacity of theobtained battery is regulated by the positive electrode. Thus, thetheoretical capacity of the battery can be obtained by multiplying thecapacity per unit weight of the positive electrode active material(LiCoO₂) (142 mAh/g), by the amount of the positive electrode activematerial included in the positive electrode active material layer.

EXAMPLE 2

In this Example, a porous heat-resistant layer comprising anonconductive filler (alumina) and a binder (polyvinylidene fluoride)was used.

A paste for forming a porous heat-resistant layer was prepared by mixing3000 g of alumina powder with a median size of 2 μm and a tap density of1.2 g/ml, 1000 g of an NMP solution of polyvinylidene fluoride (#1320manufactured by Kureha Corporation, solid content 12 wt %), and anappropriate amount of NMP, with a double-armed kneader. The paste wasapplied on both negative electrode active material layers with a diecoater. The thickness of the applied paste was set to 14 μm. Afterwards,the paste was dried by a hot blast of 130° C. at a speed of 2 m/min forfour minutes. At this time, the porosity of the porous heat-resistantlayer was set to be 60%. The negative electrode plate including theporous heat-resistant layer was cut to give the same size as in Example1, to obtain a negative electrode. A battery of Example 2 was made inthe same manner as Example 1, except that this negative electrode wasused. The amount of the binder to be added was set to 4 parts by weightper 100 parts by weight of the nonconductive filler.

EXAMPLES 3 TO 7

A separator was disposed between the porous heat-resistant layer and thepositive electrode, and the separator thickness was set to 7 μm, 10 μm,20 μm, 35 μm, and 40 μm. The ratio of the thickness of the porousheat-resistant layer to each separator thickness was 2, 1.4, 0.7, 0.4,and 0.35.

The thickness of the battery case was set to 5.2 mm, 5.3 mm, 5.6 mm, 6.0mm, and 6.2 mm in accordance with the separator thickness.

Batteries of Examples 3 to 7 were made in the same manner as Example 2except for the above. For the separator, a micro-porous film ofpolyethylene was used.

EXAMPLES 8 TO 11

The median size of alumina included in the porous heat-resistant layer 3was set to 0.3 μm, 0.5 μm, 3 μm, and 4 μm, so that the porosity of theporous heat-resistant layer was set to 35%, 40%, 66%, and 73%. Batteriesof Examples 8 to 11 were made in the same manner as Example 5 except forthe above.

EXAMPLES 12 TO 13

The speed of the hot blast for drying the paste for forming the porousheat-resistant layer was set to 5 m/min and 7 m/min, so that theporosity of the porous heat-resistant layer was 80% and 85%. Batteriesof Examples 12 to 13 were made in the same manner as Example 5 exceptfor the above.

EXAMPLE 14

The total thickness of the positive electrode was changed to 225 μm, andthe length of the positive electrode active material layer was changedto 169 mm (the area of the positive electrode active material layer: 92cm²). The total thickness of the negative electrode was changed to 227μm, and the length of the negative electrode active material layer waschanged to 387 mm. The thickness of the battery case was changed to 5.4mm. A battery of Example 14 was made in the same manner as Example 5,except for the above.

EXAMPLE 15

The total thickness of the positive electrode was changed to 190 μm, andthe length of the positive electrode active material layer was changedto 211 mm (the area of the positive electrode active material layer: 114cm²). The total thickness of the negative electrode was changed to 213μm, and the length of the negative electrode active material layer waschanged to 281 mm. The thickness of the battery case was changed to 4.9mm. A battery of Example 15 was made in the same manner as Example 5except for the above.

EXAMPLE 16

The total thickness of the positive electrode was changed to 50 μm, andthe length of the positive electrode active material layer was changedto 1020 mm (the area of the positive electrode active material layer:549 cm²). The total thickness of the negative electrode was changed to51 μm, and the length of the negative electrode active material layerwas changed to 1080 mm. The thickness of the battery case was changed to7.1 mm. A battery of Example 16 was made in the same manner as Example 5except for the above.

EXAMPLE 17

The total thickness of the positive electrode was changed to 48 μm, andthe length of the positive electrode active material layer was changedto 1060 mm (the area of the positive electrode active material layer:572 cm²). The total thickness of the negative electrode was changed to49 μm, and the length of the negative electrode active material layerwas changed to 1120 mm. The thickness of the battery case was changed to7.2 mm. A battery of Example 17 was made in the same manner as Example 5except for the above.

EXAMPLES 18 TO 22

Batteries of Examples 18 to 22 were made in the same manner as Example5, except that the amount of polyvinylidene fluoride included in theporous heat-resistant layer was set to 0.3 parts by weight, 0.5 parts byweight, 1.5 parts by weight, 7 parts by weight, and 8.5 parts by weightper 100 parts by weight of alumina.

EXAMPLE 23

A battery of Example 23 was made in the same manner as Example 5, exceptthat the binder included in the porous heat-resistant layer was changedfrom polyvinylidene fluoride to modified acrylic rubber (BM-720Hmanufactured by Zeon Corporation).

EXAMPLES 24 TO 27

Batteries of Examples 24 to 27 were made in the same manner as Example5, except that the nonconductive filler included in the porousheat-resistant layer was changed from alumina to silica, magnesia,titania, and zirconia. The median size of silica, magnesia, titania, andzirconia was set to 2 μm.

EXAMPLE 28

A battery of Example 28 was made in the same manner as Example 5, exceptthat the surface of the positive electrode was sandpapered to give asurface roughness Ra of 1.1 μm, which is rougher than the surfaceroughness (Ra=0.4 μm) of the porous heat-resistant layer. The surfaceroughness Ra of the positive electrode active material layer in thebattery of Example 5 was 0.3 μm.

COMPARATIVE EXAMPLE 1

A battery of Comparative Example 1 was made in the same manner asExample 5, except that the porous heat-resistant layer was not providedon the negative electrode.

COMPARATIVE EXAMPLE 2

A battery of Comparative Example 2 was made in the same manner asExample 5, except that the median size of alumina included in the porousheat-resistant layer was changed to 0.25 μm to give the porousheat-resistant layer with the porosity of 28%.

COMPARATIVE EXAMPLE 3

A battery of Comparative Example 3 was made in the same manner asExample 5, except that the speed of the hot blast for drying the pastefor forming the porous heat-resistant layer was changed to 8 m/min togive the porous heat-resistant layer with the porosity of 89%.

Tables 1 to 3 show the following: the formation of the porousheat-resistant layer used for the batteries in Examples 1 to 28 andComparative Examples 1 to 3; kind and the median size of thenonconductive filler; kind of the binder; the amount of the binder addedper 100 parts by weight of nonconductive filler; the area of thepositive electrode active material layer; the thickness B, porosity, andpore volume of the porous heat-resistant layer; pore volume/theoreticalcapacity; and the separator thickness A and the ratio B/A. As to thepore volume and the pore volume/theoretical capacity, the value per aside of the negative electrode, and the value per both sides of thenegative electrode are shown. The thickness B of the porousheat-resistant layer is the thickness of the porous heat-resistant layerper a side of the negative electrode.

In batteries of Examples 1 to 28 and Comparative Examples 1 to 3, thetheoretical capacity was set to the same.

TABLE 1 Porous heat- median Binder resistant size of Amount layer Kindof filler (parts by formation filler (μm) Kind weight) Ex. 1 Aramidsheet — — — — Ex. 2 Filler + Binder alumina 2 PVDF 4 Ex. 3 Filler +Binder alumina 2 PVDF 4 Ex. 4 Filler + Binder alumina 2 PVDF 4 Ex. 5Filler + Binder alumina 2 PVDF 4 Ex. 6 Filler + Binder alumina 2 PVDF 4Ex. 7 Filler + Binder alumina 2 PVDF 4 Ex. 8 Filler + Binder alumina 0.3PVDF 4 Ex. 9 Filler + Binder alumina 0.5 PVDF 4 Ex. 10 Filler + Binderalumina 3 PVDF 4 Ex. 11 Filler + Binder alumina 4 PVDF 4 Ex. 12 Filler +Binder Alumina 2 PVDF 4 Ex. 13 Filler + Binder Alumina 2 PVDF 4 Ex. 14Filler + Binder Alumina 2 PVDF 4 Ex. 15 Filler + Binder Alumina 2 PVDF 4Ex. 16 Filler + Binder Alumina 2 PVDF 4 Ex. 17 Filler + Binder Alumina 2PVDF 4 Ex. 18 Filler + Binder Alumina 2 PVDF 0.3 Ex. 19 Filler + BinderAlumina 2 PVDF 0.5 Ex. 20 Filler + Binder Alumina 2 PVDF 1.5 Ex. 21Filler + Binder Alumina 2 PVDF 7 Ex. 22 Filler + Binder Alumina 2 PVDF8.5 Ex. 23 Filler + Binder Alumina 2 Modified 4 acrylic rubber Ex. 24Filler + Binder Silica 2 PVDF 4 Ex. 25 Filler + Binder Magnesia 2 PVDF 4Ex. 26 Filler + Binder Titania 2 PVDF 44 Ex. 27 Filler + Binder Zirconia2 PVDF 4 Ex. 28 Filler + Binder Alumina 2 PVDF 4 Comp. — — — — Ex. 1Comp. Filler + Binder Alumina 0.25 PVDF 4 Ex. 2 Comp. Filler + BinderAlumina 2 PVDF 4 Ex. 3

TABLE 2 Area of positive Pore volume electrode active Porousheat-resistant layer V1/theoretical Separator material layer Thickness BPorosity pore volume capacity thickness (cm²) (μm) (%) V1* (ml) (ml/Ah)A (μm) B/A Ex. 1 183 14 60 0.1537 0.1787 — — Ex. 2 183 14 60 0.15370.1787 — — Ex. 3 183 14 60 0.1537 0.1787 7 2.00 Ex. 4 183 14 60 0.15370.1787 10 1.40 Ex. 5 183 14 60 0.1537 0.1787 20 0.70 Ex. 6 183 14 600.1537 0.1787 35 0.40 Ex. 7 183 14 60 0.1537 0.1787 40 0.35 Ex. 8 183 1435 0.0897 0.1043 20 0.70 Ex. 9 183 14 40 0.1025 0.1192 20 0.70 Ex. 10183 14 66 0.1691 0.1966 20 0.70 Ex. 11 183 14 73 0.1870 0.2175 20 0.70Ex. 12 183 14 80 0.2050 0.2383 20 0.70 Ex. 13 183 14 85 0.2178 0.2532 200.70 Ex. 14 92 14 60 0.0773 0.0899 20 0.70 Ex. 15 114 14 60 0.09580.1113 20 0.70 Ex. 16 549 14 60 0.4612 0.5362 20 0.70 Ex. 17 572 14 600.4805 0.5587 20 0.70 Ex. 18 183 14 60 0.1537 0.1787 20 0.70 Ex. 19 18314 60 0.1537 0.1787 20 0.70 Ex. 20 183 14 60 0.1537 0.1787 20 0.70 Ex.21 183 14 60 0.1537 0.1787 20 0.70 Ex. 22 183 14 60 0.1537 0.1787 200.70 Ex. 23 183 14 60 0.1537 0.1787 20 0.70 Ex. 24 183 14 60 0.15370.1787 20 0.70 Ex. 25 183 14 60 0.1537 0.1787 20 0.70 Ex. 26 183 14 600.1537 0.1787 20 0.70 Ex. 27 183 14 60 0.1537 0.1787 20 0.70 Ex. 28 18314 60 0.1537 0.1787 20 0.70 Comp. 183 — — — — 20 — Ex. 1 Comp. 183 14 280.0717 0.0834 20 0.70 Ex. 2 Comp. 183 14 89 0.2280 0.2651 20 0.70 Ex. 3*per a side of the negative electrodeEvaluation

The batteries are evaluated as follows.

Drop Resistance Test

In the batteries of Examples 1 to 28 and Comparative Examples 1 to 3, 20batteries for each Example, were dropped from a height of 2 m to aconcrete floor. The drop was carried out consecutively for 15 times. Thedrop was carried out so that the battery cover side crashes on thefloor.

Afterwards, an X-ray transmission method was used to determine thepresence or absence of a misalignment of the positive electrode and thenegative electrode in the electrode assembly and a breakage of porousheat-resistant layer. As to the misalignment of the positive electrodeand the negative electrode, the number of the battery in which thepositive electrode went out of position from the negative electrode wasdetermined. As to the breakage of the porous heat-resistant layer, thenumber of the battery which showed even a partial breakage of the porousheat-resistant layer observable was determined. The results are shown inTable 3.

Life Test

The following life test was carried out for each battery, under thecondition that both ends (in the thickness direction) of the battery wassandwiched by a stainless steel plate with a thickness of 20 mm, and aload of 17 kgf/cm² was applied on each end.

Preliminary charge and discharge was carried out, first. To be specific,a cycle of charge and discharge, at a current of 430 mA and with achanging battery voltage in a range of 3.0 to 4.1 V, was carried outtwice.

Afterwards, the battery was charged at a current of 430 mA until thebattery voltage reached 4.1 V. The battery after the charge was aged for7 days at 45° C.

A first charge and discharge cycle was repeated 500 times for the agedbattery. In the first charge and discharge cycle, a load of 17 kgf/cm²was applied as in the above, the battery voltage was changed in therange of 3.0 to 4.2 V at the current of 860 mA. The ratio of thedischarge capacity at 500^(th) cycle to the discharge capacity at thefirst cycle after the aging (initial discharge capacity) was obtained bypercentage, and regarded as a capacity retention rate. The capacityretention rate was used as a criterion for cycle life characteristic.The results are shown in Table 3.

TABLE 3 Number of Pore volume Pore volume battery in which Number ofV2** of porous V2/theoretical capacity active material battery in whichheat-resistant capacity retention layer breakage misalignment layer (ml)(ml/Ah) rate (%) occurred occurred Ex. 1 0.3074 0.3574 80 6 1 Ex. 20.3074 0.3574 81 4 1 Ex. 3 0.3074 0.3574 85 5 2 Ex. 4 0.3074 0.3574 85 21 Ex. 5 0.3074 0.3574 86 0 1 Ex. 6 0.3074 0.3574 74 0 1 Ex. 7 0.30740.3574 60 0 2 Ex. 8 0.1793 0.2086 64 0 1 Ex. 9 0.2050 0.2384 71 0 1 Ex.10 0.3382 0.3932 82 1 2 Ex. 11 0.3740 0.4350 83 4 1 Ex. 12 0.4100 0.476682 2 1 Ex. 13 0.4356 0.5064 82 4 2 Ex. 14 0.1546 0.1798 66 0 1 Ex. 150.1916 0.2226 72 0 1 Ex. 16 0.9224 1.0724 72 0 2 Ex. 17 0.9610 1.1174 650 2 Ex. 18 0.3074 0.3574 83 5 1 Ex. 19 0.3074 0.3574 82 2 1 Ex. 200.3074 0.3574 83 1 2 Ex. 21 0.3074 0.3574 73 0 1 Ex. 22 0.3074 0.3574 680 1 Ex. 23 0.3074 0.3574 80 0 2 Ex. 24 0.3074 0.3574 82 0 1 Ex. 250.3074 0.3574 80 0 2 Ex. 26 0.3074 0.3574 81 0 1 Ex. 27 0.3074 0.3574 830 2 Ex. 28 0.3074 0.3574 84 0 0 Comp. — — 54 0 4 Ex. 1 Comp. 0.14340.1668 51 0 1 Ex. 2 Comp. 0.4560 0.5302 86 9 2 Ex. 3 **per both sides ofthe negative electrode

In the battery of Comparative Example 1, in which the porousheat-resistant layer was not used and only the separator of polyethylenewas disposed between the positive electrode 1 and the negative electrode2, the capacity retention rate dropped remarkably. This is probablybecause when the battery is repeatedly charged and discharged under aload and under a condition that the charge of the battery case size isrestrained, the electrode assembly is excessively loaded to force outthe non-aqueous electrolyte from the separator.

In the battery of Comparative Example 2 as well, in which the porosity(pore volume) of the porous heat-resistant layer was excessively small,the capacity retention rate at the time of restraining the electrodeassembly deformation was low, being the same level as that of thebattery of Comparative Example 1. This is probably because when the porevolume of the porous heat-resistant layer is excessively small, theporous heat-resistant layer cannot keep the sufficient amount of thenon-aqueous electrolyte for the battery to function, relative to theforce generated from the restraining of the electrode assemblydeformation.

In the battery of Comparative Example 3, in which the pore volume of theporous heat-resistant layer is excessively large, the number of thebattery with the occurrence of the porous heat-resistant layer breakagewas large at the drop test. When the pore volume of the porousheat-resistant layer was excessively large, the structural strength ofthe porous heat-resistant layer drastically drops, to probably cause theporous heat-resistant layer to become unable to keep the shape while thebattery is in use.

On the other hand, in the battery of each of the Examples in which theporous heat-resistant layer 3 with an appropriate pore volume wasdisposed between the positive electrode and the negative electrode,cycle life characteristic and drop-resistance were improved remarkably.

Comparison of each Example is made below.

In the battery of Example 1, in which an aramid sheet was used as theporous heat-resistant layer, the porous heat-resistant layer tended tobreak at the time of the drop, though not as much as the battery ofComparative Example 3. This is probably because of a slightly weakstructural strength of the aramid sheet, and because the aramid sheetwas not attached to the active material layer of any of the electrodes.

In the battery of Example 2, in which the porous heat-resistant layerwas formed of a nonconductive filler and a binder, the number of thebattery in which the porous heat-resistant layer breakage occurred wasdecreased at the time of the drop-test. This is probably because notonly the binder causes strong binding effects between the particles ofthe nonconductive filler, but also the binder can attach the porousheat-resistant layer to the active material layer.

In the batteries of Examples 3 to 7, in which the porous heat-resistantlayer and a separator comprising polyethylene were used, the number ofthe battery in which the porous heat-resistant layer breakage occurredwas decreased at the time of the drop-test, compared with the battery ofExample 2. This is probably because the highly stiff but fragile porousheat-resistant layer was protected by the separator.

The effect of the protection of the porous heat-resistant layer by theseparator was notable when the ratio of the thickness B of porousheat-resistant layer to the thickness A of the separator, B/A was 2 orbelow, especially when 1.4 or below. Even when the separator had a largethickness, and the ratio B/A was 0.35, the capacity retention rateshowed an excellent value, and the number of the battery in which theporous heat-resistant layer breakage and the misalignment occurred wassmall. When the ratio B/A was below 0.4, cycle life characteristic wasreduced slightly, compared with the batteries of Examples 3 to 6. Thisis probably because when the size change in the battery was restrained,the separator could not fully exhibit the ability to keep thenon-aqueous electrolyte, and affected as a mere resistant component inbattery reaction.

The above results show that the ratio B/A is preferably 0.35 to 2, andfurther preferably 0.4 to 1.4.

Among the batteries of Examples 8 to 11, in which the median size ofalumina included in the porous heat-resistant layer was changed and theporosity of the porous heat-resistant layer was changed, it was foundthat in the battery of Example 8 including the nonconductive filler withthe median size of 0.3 μm, cycle life characteristic was decreasedslightly. This is probably because the excessively small median size ofthe nonconductive filler caused the nonconductive filler to be chargedtoo densely, and decreased the porosity of the porous heat-resistantlayer 3 to 35%. On the other hand, in the battery of Example 11, inwhich the nonconductive filler had the median size of 4 μm, the numberof the battery with the porous heat-resistant layer breakage wasslightly large at the time of the drop-test. This is probably becausethe excessively large nonconductive filler caused the charging state ofthe nonconductive filler rough, more than it can be assumed from theactual porosity, causing the structural strength of the porousheat-resistant layer to be reduced.

The above results show that the median size of the nonconductive filleris preferably 0.5 to 3 μm.

From the results of the batteries of Examples 12 to 13, in which thedrying conditions for the paste for forming the porous heat-resistantlayer were changed to adjust the porosity of the porous heat-resistantlayer, it was found that in the battery of Example 12, in which theporosity of the porous heat-resistant layer was 85%, the number of thebattery with the porous heat-resistant layer breakage was slightlyincreased at the time of the drop-test. The above results and theresults from the Examples 8 to 11 show that the porosity of the porousheat-resistant layer is preferably 40 to 80%.

From the results of Examples 14 to 17, in which the area of the negativeelectrode active material layer (the area of the porous heat-resistantlayer) was changed to adjust the ratio of the pore volume V of theporous heat-resistant layer to the theoretical capacity C of thebattery, it was found that when the ratio of the pore volume V1 of theporous heat-resistant layer to the theoretical capacity C (per a side ofthe negative electrode) is below 0.0899 ml/Ah, i.e., the ratio of thepore volume V2 of the porous heat-resistant layer to the theoreticalcapacity C (per both sides of the negative electrode) was below 0.18ml/Ah, cycle life characteristic was reduced slightly. This is probablybecause the amount of the porous heat-resistant layer capable of keepingthe non-aqueous electrolyte was reduced. On the other hand, when theratio of the pore volume V1 of the porous heat-resistant layer to thetheoretical capacity C (per a side of the negative electrode) is over0.57 ml/Ah, i.e., when the ratio of the pore volume V2 of the porousheat-resistant layer to the theoretical capacity C (per both sides ofthe negative electrode) is over 1.117 ml/Ah, cycle life characteristicwas slightly reduced. When the pore volume of the porous heat-resistantlayer was too large, the pores without the non-aqueous electrolytefilled hold a majority in the porous heat-resistant layer. Thus,reaction resistance of the porous heat-resistant layer to the batteryreaction increased, probably causing a slight decrease in cycle lifecharacteristic.

The above results show that the ratio V2/C is preferably 0.18 to 1.117ml/Ah.

From the results of the batteries of Examples 18 to 22, in which theamount of PVDF included in the porous heat-resistant layer was changed,it was found that when the amount of PVDF was 0.5 part by weight per 100parts by weight of nonconductive filler, because the amount of thebinder was too small, the number of the battery with the porousheat-resistant layer breakage increased slightly at the time of thedrop-test.

On the other hand, when the amount of PVDF was over 7 parts by weightper 100 parts by weight of the nonconductive filler, cycle lifecharacteristic was reduced slightly. This is probably because the binderincluded in the porous heat-resistant layer swelled by absorbing thenon-aqueous electrolyte after the battery formation, and as a result,the pore volume in the porous heat-resistant layer became small and ionconductivity was decreased.

The above results show that the amount of the binder included in theporous heat-resistant layer is preferably 0.5 to 7 parts by weight per100 parts by weight of the nonconductive filler.

The results of Example 23 show that excellent cycle life characteristicand drop-resistance can be obtained even when the kind of the binder waschanged from PVDF to modified acrylic rubber. This modified acrylicrubber also exhibits high binding effects similar to that of PVDF with asmall amount, when used as a binder.

On the other hand, when the porous heat-resistant layer includes themixture of particulate modified acrylic rubber (BM-500B manufactured byZeon Corporation) and PVDF (weight ratio 1:1) as a binder, and theamount of the binder is 4 parts by weight per 100 parts by weight ofalumina, the porous heat-resistant layer showed sufficient adhesion tothe negative electrode active material layer. However, when the porousheat-resistant layer includes the mixture of polytetrafluoroethylene andCMC (weight ratio 1:1) as a binder, and the amount of the binder is 4parts by weight per 100 parts by weight of alumina, sufficient adhesioncould not be obtained between the porous heat-resistant layer and thenegative electrode active material layer.

Thus, the binder used for the porous heat-resistant layer 3 preferablyincludes at least one of PVDF and modified acrylic rubber.

From the results of the batteries of Examples 24 to 27, in which thekind of the nonconductive filler was changed, it was found that theequivalent results with the case when alumina was used as thenonconductive filler could be obtained even when silica, magnesia,titania, or zirconia was used.

In the battery of Example 28, in which the surface roughness of thepositive electrode active material layer was made large, themisalignment of the positive electrode and the negative electrode at thetime of the drop-test could be improved remarkably. Since the porousheat-resistant layer is highly stiff but fragile, the relative positionof the positive electrode and the negative electrode tends to becomeslightly misaligned due to drops or the like. However, due to the anchoreffects by increasing the surface roughness of the positive electrodeactive material layer, the porous heat-resistant layer can be caughtinto the positive electrode active material layer. Thus, themisalignment of the relative position of the positive electrode and thenegative electrode due to the drop probably was improved remarkably.

In Examples below, battery modules as shown in FIG. 4 were made.

EXAMPLES 29 TO 33

A battery module as shown in FIG. 4 was made as in the following byusing 20 lithium ion secondary batteries in Example 5.

Each battery was connected in series by a connective terminal to obtaina stack. Then, at both ends of the stack in the battery thicknessdirection, end plates of aluminum with a thickness of 1.7 mm wereplaced. The two end plates were connected by four bridges, to bind the20 batteries. At this time, to each battery included in the batterymodule, a load of 17 kgf/cm² was applied to the ends of the battery inthe thickness direction thereof.

A battery module of Example 29 was thus made. Since the battery ofExample 5 had a thickness α of 5.6 mm, and the thickness Y of the endplate was 1.7 mm, the ratio Y/α was 0.3.

Battery modules of Examples 30 to 33 were made in the same manner asExample 29, except that the thickness of the end plate was set to 2.2mm, 6.7 mm, 11.2 mm, and 14.0 mm. In the battery modules of Examples 30to 33, the ratio Y/α was 0.4, 1.2, 2.0, and 2.5.

Evaluation

The above first charge and discharge cycle was carried out 500 times forthe battery module of each Example. The ratio of the discharge capacityat the 500th cycle to the discharge capacity at the first cycle is shownby percentage, to regard it as the capacity retention rate. Thiscapacity retention rate was used as a criterion for cycle lifecharacteristic. The size change in the battery module was checked aswell. The results are shown in Table 4.

TABLE 4 Capacity Ratio retention Size change Y/α rate (%) in module Ex.29 0.3 83 Changed Ex. 30 0.4 82 Slightly changed Ex. 31 1.2 79 No changeEx. 32 2.0 75 No change Ex. 33 2.5 69 No change

The results of Table 4 show that by using the battery of the presentinvention, the reduction in cycle life characteristic of the batterymodule can be eased.

However, in the battery module of Example 29, with the ratio Y/α ofbelow 0.4, the size change could be observed visually. In the batterymodule of Example 33, with the ratio Y/α of over 2.0, cycle lifecharacteristic was slightly reduced. Such reduction is probably becausewhen the end plates are excessively thick, the distortion from theelectrode assembly deformation could not be eased between the stackedbatteries. Thus, the above results show that the ratio Y/a is preferably0.4 to 2.

In Examples below, battery modules as shown in FIG. 6 were made.

EXAMPLES 34 TO 38

A battery module comprising a stack including a first row and a secondrow was made. In the stack, 40 lithium ion secondary batteries ofExample 5 were used to arrange each set of 20 batteries to be stacked inthe thickness direction thereof.

A stack comprising the first row and the second row was obtained, byarranging each battery in the thickness direction thereof to form a rowwith 20 batteries. In the obtained stack, a gap with a predeterminedwidth was provided between the first row and the second row.

Then, on both ends of the stack in the battery thickness direction, endplates of aluminum with a thickness of 1.7 mm were placed. At the centerportion of each end plate, two holes were provided in series in theheight direction of the plate.

A threaded shaft was inserted into each hole provided at the centerportion of the end plate, allowed to go through between the first rowand the second row, and fixed with a nut. The center portions in the twoend plates were thus connected by two connectors. Further, by using twobridges provided at both ends of the stack in the width directionthereof, two end plates were further connected. At this time, in eachbattery included in the battery module, a load of 17 kgf/cm² was appliedto the ends of the battery in the thickness direction thereof.

As in the above, the plurality of batteries included in the stack werebound by the end plates, bridges and connectors, to produce a batterymodule of Example 34. In this battery module as well, the ratio of thethickness Y of the end plate to the thickness α of the battery (Y/α) was0.3.

Battery modules of Examples 35 to 38 were made in the same manner asExample 34, except that the thickness of the end plate was set to 2.2mm, 6.7 mm, 11.2 mm, and 14.0 mm. In the battery modules of Examples 30to 33, the ratio Y/α was 0.4, 1.2, 2.0, and 2.5.

Evaluation

The above first charge and discharge cycle was carried out 500 times forthe battery module in each Example. The ratio of the discharge capacityat the 500th cycle to the discharge capacity at the first cycle is shownby percentage, to regard it as the capacity retention rate. Thiscapacity retention rate was used as a criterion for cycle lifecharacteristic. The size change in the battery module was checked aswell. The results are shown in Table 5.

TABLE 5 Capacity Ratio retention Size change Y/α rate (%) in module Ex.34 0.3 84 Changed Ex. 35 0.4 81 Slightly changed Ex. 36 1.2 80 Slightlychanged Ex. 37 2.0 76 No change Ex. 38 2.5 74 No change

The results of Table 5 show that by using the battery of the presentinvention, reduction in cycle life characteristic can be reduced even inthe case where the battery module has a stack in which two batteries arearranged in the battery width direction.

Table 5 also shows that the ratio of the thickness Y of the end plate tothe thickness α of the battery, Y/α, is preferably 0.4 to 2.

According to the present invention, even under the environment where thesize change in the battery is unacceptable, high capacity non-aqueouselectrolyte secondary batteries with excellent cycle lifecharacteristic, and battery modules comprising a plurality of suchbatteries can be provided. Such batteries and battery modules can beused as a power source for devices requiring a high output, such as HEVsand electrically-powered tools.

Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artto which the present invention pertains, after having read the abovedisclosure. Accordingly, it is intended that the appended claims beinterpreted as covering all alterations and modifications as fall withinthe true spirit and scope of the invention.

1. A battery module comprising: (a) a stack in which at least two non-aqueous electrolyte secondary batteries are stacked in at least a thickness direction thereof; (b) end plates placed at both ends of said stack, said both ends being the ends in the thickness direction of each of said batteries; and (c) at least two bridges for connecting one of said end plates with the other of said end plates to bind said batteries, wherein each of said batteries comprises an electrode assembly, a non-aqueous electrolyte, and a substantially rectangular battery case for housing said electrode assembly and said non-aqueous electrolyte, said battery case having a thickness α, a width β, and a height γ satisfying a relation α<β≦γ: wherein said electrode assembly includes a positive electrode, a negative electrode, and a porous heat-resistant layer disposed between said positive electrode and said negative electrode; said positive electrode includes a positive electrode active material layer and said negative electrode includes a negative electrode active material layer; a ratio of a pore volume in a predetermined area of said porous heat-resistant layer to a theoretical capacity of each of said batteries is 0.18 to 1.117 ml/Ah, said predetermined area having a same area as said positive electrode active material layer; a porosity of said porous heat-resistant layer is 35 to 85%, said porous heat-resistant layer includes a porous sheet comprising a heat-resistant resin; a thickness of said porous heat-resistant layer is 2 to 20 μm; and a ratio of a thickness Y of said end plate to a thickness α of the battery case, Y/α, is 0.4 to
 2. 2. The battery module in accordance with claim 1, wherein said heat-resistant resin comprises at least one selected from the group consisting of aramid with a melting temperature of 250° C. or more and polyamideimide resin with a melting temperature of 250° C. or more.
 3. The battery module in accordance with claim 1, wherein each of said batteries further includes a separator comprising a resin and being disposed between said positive electrode and said negative electrode.
 4. The battery module in accordance with claim 1, wherein a ratio of a thickness B of said porous heat-resistant layer to a thickness A of said separator, B/A, is 0.35 to
 2. 5. The battery module in accordance with claim 1, wherein said porous heat-resistant layer is attached to at least one of said positive electrode active material layer and said negative electrode active material layer.
 6. The battery module in accordance with claim 1, wherein said porous heat-resistant layer is attached to one of said positive electrode active material layer or said negative electrode active material layer, and the other of said positive electrode active material layer or said negative electrode active material layer to which said porous heat-resistant layer is not attached has a larger degree of surface roughness than that of said porous heat-resistant layer.
 7. A battery module comprising: (a) a stack in which at least two non-aqueous electrolyte secondary batteries are stacked in at least a thickness direction thereof; (b) end plates placed at both ends of said stack, said both ends being the ends in the thickness direction of each of said batteries; and (c) at least two bridges for connecting one of said end plates with the other of said end plates at peripheral positions of said end plates excluding corners thereof, to bind said batteries, wherein each of said batteries comprises an electrode assembly, a non-aqueous electrolyte, and a substantially rectangular battery case for housing said electrode assembly and said non-aqueous electrolyte, said battery case having a thickness α, a width β, and a height γ satisfying a relation α<β≦γ: wherein said electrode assembly includes a positive electrode, a negative electrode, and a porous heat-resistant layer disposed between said positive electrode and said negative electrode; said positive electrode includes a positive electrode active material layer and said negative electrode includes a negative electrode active material layer; a ratio of a pore volume in a predetermined area of said porous heat-resistant layer to a theoretical capacity of each of said batteries is 0.18 to 1.117 ml/Ah, said predetermined area having a same area as said positive electrode active material layer; a porosity of said porous heat-resistant layer is 35 to 85%, said porous heat-resistant layer includes a porous sheet comprising a heat-resistant resin; and a thickness of said porous heat-resistant layer is 2 to 20 μm.
 8. The battery module in accordance with claim 7, wherein each of said bridges is contacting one end of said stack in the width direction of each of said batteries.
 9. The battery module in accordance with claim 7, wherein a ratio of a thickness Y of said end plate to a thickness a of the battery case, Y/α, is 0.4 to
 2. 10. The battery module in accordance with claim 7, wherein said stack, end plates and bridges are configured such that a load of 17 to 100 kgf/cm² is applied to the ends of each of said batteries in the thickness direction thereof. 